The invention relates to a capacitor comprising a dielectric of a dielectric ceramic composition, which includes a doped barium-calcium-zirconium-titanate, and at least two electrodes. The invention particularly relates to a multilayer capacitor having inner electrodes of base metals.
Ceramic multilayer capacitors are customarily manufactured by means of layers of a green dielectric ceramic composition for the dielectric and layers of a metal paste for the inner electrodes which are alternately arranged one on top of the other, and finally this stack of ceramic and metal layers is subjected to a sintering operation.
The quality of ceramic multilayer capacitors is determined by the chemical composition of the material for the dielectric and for the electrodes as well as by the manufacturing conditions. In the manufacturing process, particularly the sintering conditions play an important part. Dependent upon the sintering atmosphere, various opposed oxidation and reduction reactions may take place during sintering. If sintering takes place in a reducing atmosphere, barium titanate and its derivatives, for example, the doped barium-calcium-zirconium-titanates, become semiconductive. In this condition, they cannot be used as a dielectric. Multilayer capacitors can only be sintered under oxidizing conditions if the electrode material is composed of rhodium, palladium or platinum. However, rhodium and platinum are very expensive, i.e. their share of the manufacturing cost may be as much as 50%. Therefore, the trend is towards replacing rhodium and platinum by the much cheaper nickel or its alloys. However, nickel oxidizes when sintering takes place under oxidizing conditions, so that multilayer capacitors comprising nickel electrodes must be sintered in an inert or slightly reducing atmosphere. In principle, sintering in a reducing atmosphere would cause the tetravalent titanium in barium titanate to be reduced to trivalent titanium, which leads to an extreme reduction of the insulation resistance of the capacitors. In the meantime, however, the reducibility of titanium in barium titanate has been diminished by means of additives, which are acceptors, such as Cr.sub.2 O.sub.3, Co.sub.2 O.sub.3 or MnO.
However, doping with these additives does not preclude the formation of oxygen vacancies in the crystal during sintering in a reducing atmosphere, which oxygen vacancies drastically reduce the service life of the capacitors. The oxygen vacancies exhibit a high mobility in the crystal lattice and migrate under the influence of electric voltages and temperature. As a result, the insulation resistance decreases with time.
The formation of oxygen vacancies can be partly undone when, after sintering in a reducing atmosphere, the capacitors are tempered in a slightly oxidizing atmosphere at temperatures ranging between 600 and 1100.degree. C. In this process, the oxygen vacancies in the lattice are filled up again. A disadvantage of said tempering treatment is the clear reduction of the dielectric constant .epsilon. and the negative effect on the .DELTA.C/.DELTA.t curve, i.e. the temperature-dependence of the dielectric constant.
To overcome these complex problems in the manufacture of ceramic capacitors comprising base-metal electrodes, U.S. Pat. No. 5,319,517 discloses a ceramic multilayer chip capacitor comprising inner electrodes and dielectric layers, whose dielectric material includes a dielectric oxide of the following composition: [(Ba.sub.1-x-y Ca.sub.x Sr.sub.y)O].sub.m (Ti.sub.1-x Zr.sub.x)O.sub.2, where 0.ltoreq.x.ltoreq.0.25, 0.ltoreq.y.ltoreq.0.05, 0.1.ltoreq.z.ltoreq.0.3 and 1,000.ltoreq.m.ltoreq.1,020, and to which material there is added a manganese oxide and/or a compound, which is converted into the oxide during firing, in a quantity ranging from 0.01 to 0.5% by weight, calculated with respect to oxide (MnO), an yttrium oxide and/or a compound, which is converted into the oxide during firing, in a quantity ranging from 0.05 to 0.5% by weight, calculated with respect to oxide (Y.sub.2 O.sub.3), a vanadium oxide and/or a compound, which is converted into the oxide during firing, in a quantity ranging from 0.005 to 0.3% by weight, calculated with respect to oxide (V.sub.2 O.sub.5), a tungsten oxide and/or a compound, which during firing is converted into the oxide, in a quantity ranging from 0.005 to 0.3% by weight, calculated with respect to oxide (MnO), and which comprises nickel or a nickel alloy as the material for the inner electrodes. However, the ever increasing requirements in terms of service life and reliability cannot be met, as yet, by these multilayer capacitors.